To configure an optical network using optical fibers in an optical communication field, an optical switch that switches between an incoming light and an outgoing light is necessary. In this optical fiber communication, for example, the 1.55 μm band or 1.3 μm band is used for a long-distance transmission, and the 850 nm band for a short-distance transmission.
As this type of optical switch, a crossbar type 2×2 optical changeover switch having a configuration, in which the incoming/outgoing ends of a light are provided in the linear direction (for example, Japanese Patent Laid-Open Publication No. Hei 2-100025), and a 2×2 optical changeover switch having a configuration, in which the incoming/outgoing ends of a light are provided at a right angle, are conventionally known.
A crossbar type 2×2 optical changeover switch has two sets of polarizing beam splitter to which one pair of optical fibers is connected through a collimator lens with a liquid crystal cell sandwiched between the polarizing beam splitters. This crossbar type 2×2 optical changeover switch selectively switches a light, which enters from the optical fiber of one polarizing beam splitter, and outputs the light from the optical fiber of the other polarizing beam splitter according to whether or not a voltage is applied to the liquid crystal cell.
FIG. 25 shows the configuration of an example of a 2×2 optical changeover switch in which the incoming end and the outgoing end of a light are arranged at a right angle.
Referring to FIG. 25(a), a 2×2 optical changeover switch has two optical elements on the two sides that are at aright angle to a polarizing beam splitter 101: an optical element composed of a liquid crystal cell 103 and a light reflector 105, and an optical element composed of a liquid crystal cell 104 and a light reflector 106. An optical fiber 109 is connected via a collimator 107 to the side, opposed to the liquid crystal cell 103, with the polarizing beam splitter 101 between them, and an optical fiber 110 is connected via a collimator 108 to the side, opposed to the liquid crystal cell 104, with the polarizing beam splitter 101 between them. Because light enters and exits from the optical fibers 109 and 110, circulators 111 and 112 are connected to the optical fibers 109 and 110 to separate an incoming light from an outgoing light. A voltage is applied to the liquid crystal cells 103 and 104 to change the polarization state of an incoming light by λ/4 wavelength. In this configuration, the crystal cells are combined with the light reflectors to change the polarization state by λ/2 wavelength by adding up the changes in the polarization state of both an incoming light and an outgoing light.
A 2×2 optical changeover switch having this configuration can execute two types of switching operation states: an exchanging operation state in which the incoming end and the outgoing end are different and a straight operation state in which the incoming end and the outgoing end are the same. The polarizing beam splitter 101 has a polarized light separation/combination film 102.
FIG. 25(b) is a diagram showing the exchanging state. In this operation state, no voltage is applied to the liquid crystal cell 103 and the liquid crystal cell 104. A light entering from the optical fiber 109 is composed of two polarization components p and s whose polarization planes are at a right angle. After changed to parallel beams by the collimator 107, the light enters the polarizing beam splitter 101 where it is separated by the polarized light separation/combination film 102 into the polarization component p that goes straight and the polarization component s that reflects.
When no voltage is applied to the liquid crystal cell 103 and the liquid crystal cell 104, the polarization state is changed by the liquid crystal cell 103 and the liquid crystal cell 104. In FIG. 25(b), ON indicates that the polarization state is changed. The polarization component p, which goes straight, passes through the liquid crystal cell 103 where the polarization state is changed by λ/4 wavelength, is reflected by the light reflector 105, and passes through the liquid crystal cell 103 again. At this time, the polarization state is further changed by λ/4 wavelength and the incoming polarization component p is changed to the polarization component s. The changed polarization component s is reflected by the polarized light separation/combination film 102 and is output to the optical fiber 110 via the collimator 108. On the other hand, the polarization component s that is reflected by the polarized light separation/combination film 102 passes through the liquid crystal cell 104 where the polarization state is changed by λ/4 wavelength, is reflected by the light reflector 106, and passes through the liquid crystal cell 104 again. At this time, the polarization state is further changed by λ/4 wavelength and the incoming polarization component s is changed to the polarization component p. The changed polarization component p passes through the polarized light separation/combination film 102 and is output to the optical fiber 110 via the collimator 108. In this way, the incoming light exits from the end different from the incoming end.
FIG. 25(c) is a diagram showing the straight state. In this operation state, a voltage is applied to the liquid crystal cell 103 and the liquid crystal cell 104. When a voltage is applied to the liquid crystal cell 103 and the liquid crystal cell 104, the liquid crystal cell 103 and the liquid crystal cell 104 do not change the polarization state. In FIG. 25(c), OFF indicates that the polarization state is not changed.
The polarization component p, which goes straight, passes through the liquid crystal cell 103, with the polarization state unchanged, is reflected by the light reflector 105, and passes through the liquid crystal cell 103 again with the polarization state unchanged. After reflected, the polarization component p passes through the liquid crystal cell 103, goes straight through the polarized light separation/combination film 102, and is output to the optical fiber 109 via the collimator 107. On the other hand, the polarization component s, which is reflected by the polarized light separation/combination film 102, passes through the liquid crystal cell 104 with the polarization state unchanged and, after that, is reflected by the light reflector 106 and passes through the liquid crystal cell 104 again with the polarization state unchanged. After reflected, the polarization component s that passes through the liquid crystal cell 104 is reflected by the polarized light separation/combination film 102 and is output to the optical fiber 109 via the collimator 107. This causes the incoming light to be output from the end that is the same as the incoming end.
The configuration of the 2×2 optical changeover switch described above and the configuration of an add drop multiplexer using the 2×2 optical changeover switch are described, for example, in Optical-Engineering, Vol. 40 No. 8, 1521–1528, August 2001 (Sarun Sumriddetchakajorn, Nabeel A. Riza, Deepak K. Sengupta).
It is required that the function of an optical switch be symmetrical when the incoming light and the outgoing light are exchanged. When the incoming light and the outgoing light are exchanged in the configuration of the 2×2 optical changeover switch described above, the function is not always symmetrical because the optical path, liquid crystal cells through which the light passes, and the optical system may differ according to the polarization component in the optical switch. The problem is that, in order to make the function symmetrical when the incoming light and the outgoing light are exchanged, the optical path lengths must be made equal or the characteristics of the liquid crystal cells and the optical system must be adjusted. Because more adjustment is required as the configuration requires more parts, this problem becomes more serious and affects the manufacturing cost.
A Polarization Dependent Loss (PDL) and a Polarization Mode Dispersion (PMD) are known as a loss generated by an optical signal. The polarization dependent loss is generated, for example, when the signal is transmitted through the electrodes provided on the liquid crystal cell and the signal strength is weakened. In the configuration in which the light passes through liquid crystal cells many times, the problem is that the polarization dependent loss becomes large. The problem with the polarization mode dispersion is that, because the level of optical pulse dispersion differs according to the polarization modes of components at a right angle, different-length optical paths increase the signal deterioration caused by this optical dispersion.
In addition, in a configuration in which the polarization state is switched by a liquid crystal cell, the switching speed depends on the switching speed of the liquid crystal cell. Because the switching speed of this liquid crystal cell is proportional to the square of the thickness of the liquid crystal cell, a thinner liquid crystal cell is desirable. Another problem is that, because the thickness of the liquid crystal cell depends on the polarization angle to be changed and the wavelength of the light, the high-speed operation is difficult.
To solve the prior-art problems, it is an object of the present invention to eliminate the need for the adjustment of the optical path length or the optical characteristic and to reduce signal deterioration due to the polarization mode dispersion by making the optical path length and the optical characteristics equal for the polarized lights that pass through different paths. It is another object of the present invention to reduce the polarization dependent loss by reducing the number of times the light passes through the electrode of a liquid crystal cell and to reduce the cost by using a configuration in which the number of parts and adjustment points is decreased or reduced. It is still another object of the present invention to increase the operation speed of an optical switch by increasing the response of a liquid crystal cell.